Influence of zirconium on cation mobilities in Na2O-CaO-Al2O3-SiO2 melts: A multicomponent diffusion and XANES study

Abstract

High temperature cation mobilities, based on the multicomponent diffusion method, have been coupled with a structural investigation in the Na2O-CaO-Al2O3-SiO2-ZrO2 (NCASZ) system at 1200 °C and 1250 °C. From structural investigation using X-ray absorption spectroscopy (XANES), zirconium was determined in six-fold coordinated sites in the reduced glass composition range of our study and no significant structural changes have been evidenced compared to a quaternary Na2O-CaO-Al2O3-SiO2 (NCAS) glass with a similar composition. Additionally, the diffusion experiments revealed that adding zirconium in the quaternary melt has no influence on the dominant eigenvectors that correspond to exchange between calcium and sodium. Despite the expected mobility decrease resulting from a viscosity increase upon the addition of zirconium, the dominant eigenvalue calculated in this study is higher when Zr is introduced in the melt structure. This result strongly suggests that the sodium mobility is enhanced by the presence of Zr, which is explained by a change in the structural role of sodium from network modifier associated to Q3(Si) sites in NCAS melts to charge compensator associated with [ZrO6]2− sites in NCASZ melts. This modification generates lower bond strengths between sodium and other cations in the melt, thus favoring an enhancement in mobility. Moreover, the diffusion matrix was applied to predict diffusion profiles between zirconium-bearing crystals and melts. We observed that even far from the composition field used for the matrix determination, predictions of zirconium diffusion profiles may be relevant and demonstrate the potential of this approach to evaluate crystal/melt dissolution behavior.